Attention-deficit hyperactivity disorder (ADHD) is an early-onset, clinically heterogeneous disorder of inattention, hyperactivity, and impulsivity. Family, twin, adoption, segregation analysis, and molecular genetic studies show that is has a substantial genetic component. Although their results are still tentative, molecular genetic studies suggest that three genes may increase the susceptibility to ADHD: the D4 dopamine receptor gene, the dopamine transporter gene, and the D2 dopamine receptor gene. Studies of environmental adversity have implicated pregnancy and delivery complications, marital distress, family dysfunction, and low social class. The pattern of neuropsychological deficits found in ADHD children implicate executive functions and working memory; this pattern is similar to what has been found among adults with frontal lobe damage, which suggests that the frontal cortex or regions projecting to the frontal cortex are dysfunctional in at least some ADHD children. Moreover, neuroimaging studies implicate frontosubcortical pathways in ADHD. Notably, these pathways are rich in catecholamines, which have been implicated in ADHD by the mechanism of action of stimulants--the class of drugs that effectively treats many ADHD children. Yet human studies of the catecholamine hypothesis of ADHD have produced conflicting results, perhaps due to the insensitivity of peripheral measures.